Formation and wellbore scale prevention

ABSTRACT

Disclosed herein is a method for the reduction of the scale deposition in the formation and fractures about a producing wellbore. In particular, gypsum scale deposition is avoided by the addition of scale preventative to an absorbent which may be used as a propping agent and positioned in hydraulically created fractures within the formation. Subsequent production from the formation, through the fractures, slowly bleeds the gypsum scale preventative into the produced water and thereby prevents gypsum scale deposition in the wellbore.

United States Patent 1191 Fulford Jan. 1, 1974 [54] FORMATION ANDWELLBORE SCALE 3,258,071 6/1966 Shen 166/275 PREVENTION 3,179,170 4/1965Burtch 166/279 3,528,502 9/1970 Oleen 166/279 Inventor: Richard Fulford,a, 0 3,502,587 3/1970 Stanford et al. 166 279 ux 3,483,925 12/1969Slyker 166/279 [73] Ass'gnee 82;: Cmnpany Tulsa 3,467,192 9/19 9 Nolan,111 e131. 166/279 [22] Filed: May 25, 1-972 Primary Examiner-Stephen J.Novosad [211 App] No: 256 722 Attorney-Elton F. Gunn et a].

63 Related US. Application Data 57] ABSTRACT C f N ,193, A 11,19 l 1 532 11 25? 0 Set 0 63 ug 7O Disclosed herem 1s a method for the reductionof the scale deposition in the formation and fractures about a 52 us. c1166/279, 166/280 Producing wellhore- In particular, gypsum Scale depo-511 1111.121 E2lb 43/26 shioh is avoided by the addition of ScalePreventative [58] Field 0fSearch..... 166/279, 280, 30s, to an absorbentwhich may he used as a Propping 166/307 309, 252/855 A 855 B, 855 agentand positioned in hydraulically created fractures E within theformation. Subsequent production from the formation, through thefractures, slowly bleeds the [56] References Cited gypsum scalepreventative into the produced water UNITED STATES PATENTS and therebyprevents gypsum scale deposition in the wellbore. 3,283,817 11/1966Roberts 166/279 3,481,400 I2/1969 Kerver et a1. 166/279 8 Claims, 1Drawing Figure PORE VOLUMES INJ ECTED PAIENIEUJAH H974 VISCO cowc.

( MGJL.) 8O

4 a l2 l6 2o 24 2s 32 PORE VOLUMES INJECTED BY r- FORMATION AND WELLBORESCALE PREVENTION This application is a continuation of application Ser;No. 63,193, filed Aug. ll, 1970 now abandoned.

BACKGROUNDOF THEINVENTION This invention relates to the prevention offormation of scale deposits resulting, for example from thewaterflooding of a reservoir for the secondary recovery of petroleum.More particularly, itzrelates to amethod for conditioning a. reservoir:and. the' water contained therein by the introduction of a proppingagent which is impregnated with scale. preventative so thatthepreventative is slowly bled; from the propping. agent into theproduced waters such that scale deposition does not occur in thevicinity. of the production'well.

Water injectedinto aireservoir'to producepetroleum remaining in areservoir. after'primary recovery may become supersaturated withscale,.for example calcium sulfate, as it is driven through thereservoir at high pressure. As the flood waters approachzthe'vicinity ofan oil production well, initial'pressures are diminished During thisdiminishment of pressure the calcium sulfate solubility is rapidlyreduced; At this point in the production drive cycle, calcium"sulfate-begins to precipitate out and form'scalesdeposits in thevicinity of the wellbore and in particular in the wellboreitself. Thisscale impedes the flow of fluids and:often.shuts off tliewellbore sothat petroleum production from-the'reservoir is restricted.

Prior solutions for the-preventionof scaleidepositionand for the removalthereof may-be categorized into two classes of physical and chemical.preventative:

means. The physical'rem'ovalof scale may beaccomplished by drilling thedeposited scale from the: well. This procedure is expensive andv isaccompaniedby production time lossduringthe drillinggandlsubsequentscale deposition on the-active.sites which were'drilled.

ited with gypsum scale with'the wellbore'again'shut off to the remainderof theformation.

Chemical treatmentof? production wells with phosphates orpolyacrylamides; whicharescale preventatives, hasbeen implementedtopreventscale fromini tially forming by blockingthe.active-sites forscale-deposition. Shen, US. Pat; No. 3,258,071, describes the use ofthese chemicals to minimizeprecipitation of in-- soluble metal compoundsin floodirlg waters. Chemicals are placed in the wellboreorforcedintothe'reser voir in order to prevent scalefrom precipitatingthere.

from. This method of treatment has-serious drawbacks in'that thechemical introduced into th'e wellborewill tend to mixwithfluidsat'thetop'ofthe wellboreand thereby not mix within thewellsuchth'at the fluids that enter from the bottom of the wellbore: areuntreatedi Consequently, chemicals: are relatively ineffective inpreventing scale from forming: at the bottom of the wellborewhere themost severe scalingsoften occurs; A

second problem with" chemical treatment is thatin V order to preventscale formation in the reservoir, chem icals must be introduced into theparticular zones of the reservoir in which the. scaling normally occurs.This procedure has not been successful as it is difficult to forcechemicals selectively into those zones where scaling occurs mostrapidly. Quite often, chemicals are forced into regions where scaling isnot occuring, with the chemicals being wasted or lost to the formationwith no scale prevention being derived.

What is required is a method by which flooding media may be treated suchthat the prevention of bivalent cation precipitation or the preferentiallowering of the interfacial tension of surfaces-to the cations of calcium, iron, magnesium, etc. is provided. By this treatment, theformation in the vicinity of the production well is left unobstructedand thereby no scale buildup produced therein so that no production lossor productivity of the well is incurred.

It is an object of this invention to provide an improved method for therecovery of petroleum from subterranean reservoirs during secondaryrecovery by waterflooding.

It is another object of the present invention to provide a method forpreventing the precipitation of scale near the vicinity of a productionwell during. the secondary recovery of petroleum by means ofwaterflooding.

It is still a further object of the present invention to provide amethod for preventing the formation of gypsum scale in'the vicinity of aproduction well during the secondary recovery of petroleum by theintroduction of a gypsum scale preventative in combination with aproppingagent such that continuous dissolution of the gypsum-scalepreventative from the propping agent occurs so that the flooding mediais continually treated and-no calcium sulfate deposition occurstherefrom.

With these and other objects and advantages of the present invention inmind, the invention will be more fully developed with-particularreference to the following drawingand description:

SUMMARY OF THE INVENTION The objects of the present invention areaccomplished by the application of a process for the prevention ofsulfate deposition in producing oil wells. The process comprisesimpregnating a propping agent with a scale preventative. The formationin the vicinity of the wellbore is hydraulically fractured withsubsequent positioningof the impregnated propping agent within thehydraulic fractures created. After the propping agent has beenpositioned, thewell is returned to a productioncycle with oil-and waterproduced through the newly created fracture network. Scale preventativeis bled into the producing waters from the propping agent. By thismethod the producingwaters are treated such that scale deposition doesnot occur in the vicinity of the wellboreor in the wellbore itself.

Generally, the formation is produced until the concentrationof the scalepreventative reaches a minimal level. In most'applications of themethod, this minimal BRIEF DESCRIPTIONOF THE DRAWING The presentinvention may be more fully understood 1 ..5f ft th?.fel e sdevir it Wsh;

3 The FIGURE represents the bleeding of the calcium sulfate scalepreventative from the propping agent with increasing volumes ofinjection of driving fluid.

DETAILED DESCRIPTION OF THE INVENTION Most successful field applicationsof scale preventatives have been found in sandstone formations whereinchemicals are released from the reservoir over relatively long periodsof time such that continuous prevention of the scale formation isprovided. In the application of preventatives to carbonate reservoirs,the adsorption process has not worked favorably in that the scalepreventatives do not adhere sufficiently or adsorb well upon carbonates.Investigation of the adsorption of scale prevention chemicals hasdisclosed that chemicals may be adsorbed upon particular propping agentssuch that they may be introduced into a fracture network within aformation and positioned so that subsequent production from theformation continuously bleeds or de-adsorbs the scale preventative fromthe propping agent, thereby treating the produced waters and preventingscale deposition within the formation in the vicinity of the wellboreand the wellbore itself. By this procedure scale preventative isadsorbed on a propping agent, before introduction into the well andformation. Preceding the introduction of the propping agent, the well ishydraulically fractured. A fluid containing the propping agent thereinis then introduced in the fractures so thus as the fractures settle uponthe propping agents, the scale preventative is constantly bled into thefluid which passes about the propping agent so as to inhibit gypsumscale deposition in the formation, wellbore and upon the wellboreequipment.

This bleeding of the preventative may be timed by the amount ofadsorption of scale preventative upon the propping agent. The mostprevalent forms of scale found in reservoirs are of calcium sulfate orgypsum, barium sulfate, calcium carbonate, strontium carbonate, ironoxide, iron sulfide and magnesium sulfate precipitates. Although each ofthese forms may be prevented by the process of the present invention,for the. purpose of illustration, the gypsum scale problem will bediscussed herein although the scale preventatives enumerated willequally well prevent deposition of each of these scale producingcompounds. Table l is illustrative of some of the gypsum scalepreventative chemicals which may be utilized in the present inventionand the amount of adsorption which may be maintained on sand, inparticular, and other preferred propping agent, which may achievespeciflc concentrations of the gypsum scale preventative for thetreatment. Therefore, Table 1 depicts the concentration of the solutionfor the addition of gypsum scale preventative to sand and the amount ofpreventative actually adsorbed upon the sand granules.

TABLE I Adsorption on Sand Chem. Con- Mg. adsorbed centration. TradeName Chemical Name per kg. Sand (Mg./l.) Visco 959 Organic 25 I00Phosphate 200 350 L000 500 10,000 Corcxl 7640 Organic 29 I00 Phosphate25 200 25 L000 65 LP-53 Poly Orgunic Acid 47 IS (Probably Polyucrylicacid) Dcqucst 20l0 l-Hydruxy, l,l 25 I00 Diphosphonic Acid 25 200 Ethunc25-50 1,000

Dowell L-37 50% Water Solution 22 100 of Nitrilotris (Methylene) 28 200Triphosphonic Acid 25 [,000 Pusher 500 Partially Hydrolyzed 500Polyacrylamide (25% of Amide Groups hydrolyzed to acid groups) Polymer454 Highly Cross-Linked H00 500 Polyacrylamidc To measure the amount ofadsorption and subsequent amount of gypsum scale prevention which may beobtained by the present invention, the following examples wereconducted:

EXAMPLE I Adsorption of gypsum scale preventatives was measured oncolumns of crushed dolomite rock cores which were obtained from a WestTexas well in which calcium sulfate scale deposition was prevalent. Therock was principally dolomite with small amounts of anyhydrite andshale. The columns were 20 inches long and 2 inches in diameter with 200millimeters of a 1 percent gypsum scale preventative solution placed inthe column and allowed to stand for 24 hours. During the soaking period,the gypsum scale preventative was adsorbed on the dolomite. Syntheticbrine was then injected through the column with the preventativeconcentration of the affluent measured. The brine contained 45.8 gramsper liter sodium chloride, 5 grams per liter calcium chloride and 1.9grams per liter magnesium chloride. Analysis procedures for thepreventatives were provided by the supplier chemical companies. Table 2illustrates the gypsum scale inhibition which was found by the use ofthe two chemicals tested. This inhibition is measured in percent scaleinhibition by standard measuring techniques.

TABLE 2 Chemical Percent Chemical Concentration (ppm) Scale InhibitionVisco 959 l l 93 As can be seen, Visco 959, an organic phosphate, wasfound to have excellent scale preventative properties but pooradsorption properties, while L-37, a 50 percent water solution ofnitrilotris or methylene triphosphonic acid, was found to have fairscale preventative properties, but extremely good adsorptioncharacteristics. The flow of Visco 959 from the column was very similarto that of an ammonium thiocyanate tracer which is non-adsorbed on therock. Referral to the FIG- URE will illustrate the use of the Visco 959chemical and depicts the rapid depletion of the chemicals from thedolomite and therefore the small amount of gypsum scale prevention whichwould be obtained over a prolonged period by this procedure. Therefore,the Visco 959 washed out of the column, with only a small amount ofadsorption. Although the L-37 concentration decreased very rapidly to 70parts per million, it then remained constant for over 10 pore volumesbefore its concentration began to decrease significantly. The poorgypsum scale inhibition characteristics of L-37 would inhibit the use ofit in a reservoir. Therefore, by examination of the chemical adsorptionproperties and gypsum scale prevention properties of Visco 959 and L-37,one would determine that although one adsorbs more readily and createsless gypsum scale prepropping agents which willadsorbthegypsum-scale,

preventative andallowza long periodof dissolution of the scalepreventatives' 'from the proppingagent, one will be allowed to treatawell or formation over longer periods of time and therefore'preventgypsum scale formation'for longer intervals of production thanpreviously available by the mere injection into'a' reservoir of a gypsumscale preventative.

EXAMPL-E 2 To illustrate the manyavailablegypsum scale preven- TABLE 3.-Data mites; both by trade name and. general chemical species which maybeutilized in the present invention, Table 3 is disclosed. Table 3illustrates the'concentration in parts per million of the preventative,the precipitation time in hours; the percent scale inhibition which wasobtained from the use of the gypsum scale preventativeand the amount ofthe super-saturation of the calcium sulfate in the testwater when theseinhibitors are utilized with sand as the propping agent adsorbent.Therefore, by use of Table3, one may determine a suitable gypsum scalepreventative, add this gypsum scale preventative to the propping agentand hydraulically fracture the well with hydraulic fracturing fluid inconjunction with the introduction of the propping agent and therebyobtain gypsum scale inhibition.

Calcium Precipita- Percent sulfate super- Conc. tion time scalesaturation Inhibitor; Co. name Chemical type (ppm) (hrs.) inhibition(ppm) Visco 950; Nalco Chem. Co Organic phosphate 11 3 93 6837 1 1 1 1001823 L-37; Dowell Organic phosphonate 11 2 18 2107 22 3 8 2640 5-31;Calgon Chem. Co ..do 11 3 6 6837 11 1 28 1823 Baroid H-35; Natl. Lead CoOrganic phosphate l1 1 12.5 1823 22 4 39 1754 Baroid 11-36; Natl. LeadCo ..do 11 1 12.5 1823 22 4 19 1754 Gyptrol V1; Cardinal Chem. CoOrganic phosphonate 11 3 6 6837 11 1 35 1823 6-54; Champion ..clo 1 1 35 6837 11 l 47 1823 P-262; Amer. Cyanamide........................Polyacrylamide copolymer l1 1 0 1823 22 3 12 3010 Pusher 500; Dow Chem.Co Partially hydrolyzed polyacrylamide 11 1 91 1823 22 3 82 3010 Polymer454; Calgon Chem Cross-linked partially hydrolized poly- 11 4 98 1789acrylamide. 22 4 74 1789 LP-53; Haliburton Organic liquid (polymer)polyacrylic acid... 11 1 84 1823 22 3 93 2640 L-42; Dowell 10 percentZirconium chloride in water 1 1 4 16 1789 22 4 6 1789 7637; Enjay Anionorganic liquid 22 3 days A 92 30101 30 4 days 93 2855 Dimethylphosphonate; Aldrich Chem. Co 22 3 days 2 3010 30 4 days 2 2855Dequest2041; Monsanto Ethylenediamine tetra methylene phos- 1 1 3 791749 phonicacid. 22 3 1479 Dequest 2010; Monsanto l-hydroxy, l, 1diphosphonic acid ethane... 11 3 97 1479 22 3 99 1479 Calgon D-240;Calgon Chem. Co Partially hydrolyzed polyacrylamide (simil1 3 77 2442lar to Pusher 500). 22 3 89 2442 Calgon WC 500 Partially hydrolyzedpolyacrylamide 11 3 87 2442 22 3 86 2442 TABLE 3. Data-Continued ICalcium PFBCIPIIH- Percent sulfate super- Conction time scale saturationinhibitor; Co. name Chemical type (ppm) (hrs) inhibition (ppm) DoWellJTS I f": I f. ...T. I. ..db I. l 4 65 3909 l l 3 89 2442 22 3 98 2442 54 84 3909 i 4 93 3909 25 4 95 3909 50 4 92 3909 Corext 7640; EnjayOrganic phosphate 1 l 5 94 2253 22 5 92 2253 SPl75; Tretolyte Organicphosphate l l 2 100 l85l 22 2 100 1851 A gain referring t6theidfiljilijhe second cum/ e on the FIGURE depicts the utilization ofVisco 959 with sand in a packed column. It can be discerned that therapid dissolution of the Visco 959 as observed when it is adsorbed ondolomite is curtailed and that many pore volumes of water may be passedover the propping agent before the Visco 959 gypsum scale preventativeis depleted.

Through further laboratory testing, it has been found that when aconcentration is decreased to a point wherein the gypsum scalepreventative concentration in the produced water is below about tenparts per million, the gypsum scale prevention reaches a point at whichit is ineffective for reducing gypsum scale deposition in the formationand wellbore. Therefore, it is a preferred embodiment of the presentinvention that the formafion in the vicinity of the wellbore beretreated by the process of the present invention when the concentrationof the gypsum scale preventative in the production water reaches aconcentration of the gypsum scale preventative in the production waterreaches a concentration of about less than ten parts per million gypsumscale preventative in the production water.

The present invention may be utilized in conjunction with variouspropping agents which have the adsorption property of adhering gypsumscale preventative. Typical, but not exclusive of these propping agents,would be those selected from the group consisting of sand, silica geland alumina. In particular, the use of sand has proved to be anexcellent propping agent absorbent for the gypsum scale preventative. Ithas been found that excellent gypsum scale preventatives are includedwithin the organic phosphates and polyacrylamide chemical groups andthat these gypsum scale preventatives, although not exclusive, arepreferred in the use of the process of the present invention. Althoughhigh concentrations of preventative are desired, for example up to a oneto one ratio of preventative to propping agent, the exact concentrationof preventative desirable will generally be determined for eachindividual reservoir and waterflooding situation.

Through use of the process of the present invention, scale depositionwithin the formation in the vicinity of a producing wellbore and uponthe producing wellbore equipment itself may be prevented by theintroduction of pretreated propping agent which has a calculated amountof adsorbed scale preventative contained therein. Thereby, throughcontinued production from the well, after treatment, the well iscontinually treated with scale preventative such that scale does notform and thereby block or inhibit the production of oil therefrom.Through use of the present invention, highly successful secondaryrecovery operations through the use rwwa terflooting, and especially indolomite reservoirs which previously could not be treated by scalepreventatives, may be achieved. The process has been found to be asimplified treatment for reducing scale formation in otherwise highlyproductive reservoirs such that high productivity may be obtained frompreviously in adequate production wells. The present invention provvidesa significant advance in the prevention of scale in the vicinity ofproduction wells during waterflooding operations by providing asimplified economic process by which the precipitation of scale may berestrained or totally prevented. The method may be utilized inconjunction with present waterflooding media to enhance the performancethereof in reservoirs which previously were determined unsuitable forwaterflooding operations due to uncontrollable scale formation.

While the invention has been described above with respect to certainembodiments thereof, it will be understood by those skilled in the artthat various changes and modifications may be made without departingfrom the spirit or scope of the invention as presented herein.

1 claim:

1. A process for inhibiting scale deposition in a producing oil wellwhich comprises:

a. fracturing a carbonate formation in the vicinity of a wellbore;

b. positioning a propping agent in the fissures of the fracturedformation, said propping agent being selected from the group consistingof sand, silica gel, and alumina and having an organic phosphate scaleinhibitor adsorbed directly on the surface thereof,

c. producing the oil well through the propped fissures of the formation.

2. The process of claim 1 further comprising repeating steps (a), (b)and (c) when the scale preventative concentration in the produced watersbecome less than about 10 parts per million.

3. The process of claim 1 in which the propping agent is positioned inthe fracture by simultaneously introducing it into the calcium carbonateformation with a hydraulic fracturing fluid.

l 4. The process of claim 1 in which scale is calcium sulfate.

5. A process for inhibiting scale deposition in a producing oil wellwhich comprises:

a. fracturing a carbonate formation in the vicinity of a wellbore;

b. positioning a propping agent in the fissures of the fracturedformation, said propping agent being selected from the group consistingof sand, silica gel, and alumina and having a polyacrylamide scaleinhibitor adsorbed directly on the surface thereof, c. producing the oilwell through the propped fissures 10 is positioned in the fracture bysimultaneously intro ducing it into the calcium carbonate formation witha hydraulic fracturing fluid.

8. The process of claim 5 in which the scale is calcium sulfate.

2. The process of claim 1 further comprising repeating steps (a), (b)and (c) when the scale preventative concentration in the produced watersbecome less than about 10 parts per million.
 3. The process of claim 1in which the propping agent is positioned in the fracture bysimultaneously introducing it into the calcium carbonate formation witha hydraulic fracturing fluid.
 4. The process of claim 1 in which scaleis calcium sulfate.
 5. A process for inhibiting scale deposition in aproducing oil well which comprises: a. fracturing a carbonate formationin the vicinity of a wellbore; b. positioning a propping agent in thefissures of the fractured formation, said propping agent being selectedfrom the group consisting of sand, silica gel, and alumina and having apolyacrylamide scale inhibitor adsorbed directly on the surface thereof,c. producing the oil well through the propped fissures of the formation.6. The process of claim 5 further comprising repeating steps (a), (b)and (c) when the scale preventative concentration in the produced watersbecome less than about 10 parts per million.
 7. The process of claim 5in which the propping agent is positioned in the fracture bysimultaneously introducing it into the calcium carbonate formation witha hydraulic fracturing fluid.
 8. The process of claim 5 in which thescale is calcium sulfate.